Various types of combustion may be used in an internal combustion engine. For example, spark ignition (SI) of a homogenous mixture during the expansion stroke is one example method. This method relies on a timed spark from a sparking plug in order to achieve ignition within the combustion chamber of an air and fuel mixture. Another type of combustion may be referred to as homogeneous charge compression ignition (HCCI), which occurs when the temperature of the combustion chamber exceeds autoignition temperature for the specific fuel resulting in autoignition. HCCI can be used to provide greater fuel efficiency and reduced NOx production under some conditions.
One approach to utilizing autoignition combustion is described in U.S. Pat. No. 6,293,246. In this approach, rather than relying on autoignition to initiate combustion, a spark assisted type of auto-ignition operation is utilized. Specifically, the approach in U.S. Pat. No. 6,293,246 relies on spark assist at all times in order to initiate autoignition of a mixture that has been raised to a temperature close to the autoignition temperature. In this example, the spark assisted combustion process requires the temperature of the gas within the combustion chamber attain a state near autoignition without achieving combustion. By firing a spark and initiating combustion in a portion of the combustion chamber, the pressure, and hence the temperature, may be increased in the entire combustion chamber. Thus, the gases which were near autoignition, are at or above an autoignition temperature, and autoignition occurs throughout the chamber. This phenomena is in contrast to spark ignition combustion in which a spark is fired thereby initiating a flame front which progresses through the combustion chamber into a mixture. In contrast, spark ignition combustion occurs in a mixture which is rich enough to sustain and propagate a flame front. Furthermore, the mixture is cool enough ahead of the flame front to resist autoignition. A sparking mechanism is then utilized to assist in initiating combustion within the chamber.
The inventors herein have recognized a disadvantage with such an approach. Specifically, conditions may exist, such as with low engine load, in which the spark assisted autoignition combustion generates greater NOx and reduced levels of fuel economy that may be otherwise realized through non-spark assisted HCCI combustion. Additionally, the inventors recognized that non-spark assisted combustion may be limited at high loads due to degradation of accurate temperature control within the combustion chamber.
In one approach, the above issues may be addressed by a method of operating an internal combustion engine having a combustion chamber with a piston. The method comprises, during a first mode, adjusting an operating condition of the engine so that a first mixture of air and fuel in the combustion chamber attains an autoignition temperature and combusts without requiring a spark from the spark plug; and during a second mode, adjusting said operating condition of the engine so that a second mixture of air and fuel in the combustion chamber attains, but does not achieve, said autoignition temperature; and performing a spark from the spark plug after top dead center of piston position so that said second mixture combusts.
In this way, it is possible to achieve improved fuel economy and reduced emissions during lower loads by using non-spark assisted combustion, while expanding the ability to operate at higher loads using spark-assisted autoignition. Thus, such multi-mode operation may achieve an overall improved fuel economy and lower NOx production by exploiting the advantages of each mode under alternate operating conditions.
Note that operation during a non-spark assisted condition may still utilize what may be referred to as a waste spark, where the spark plug is fired at a later point after which auto-ignition should have occurred (i.e., the spark is present to initiate combustion in cases where the auto-ignition temperature is inadvertently not attained). In this way, reliable combustion can be provided even through some deviation in the temperature control may occur and the expected auto-ignition temperature is not achieved.